Usually, a liquid crystal display device comprises a liquid crystal cell having a structure that a liquid crystal component oriented in a predetermined direction is sandwiched between two supporting substrates of glass or the like, and an optical film such as a polarizing film, a phase difference film and a luminance improving film, and a pressure-sensitive adhesive is often used when the optical films are laminated with each other or adhered to the liquid crystal cell.
The liquid crystal display device is broadly used as a display device such as a personal computer, a television and a car navigation. In accordance with this, it is needed for a pressure-sensitive adhesive to be excellent in durability even though it is used under a severe environment such as high temperature and humidity conditions, namely not to cause separation, occurrence of air bubbles or the like even though it is used for long periods of time. In addition, the optical film is subjected to a large dimensional change such as shrinkage or expansion under the severe environment such as high temperature and humidity conditions, but stress occurring due to the dimensional change can not be relaxed by the pressure-sensitive adhesive layer, so that residual stress of the optical film becomes uneven. As a result, so-called “bright defect” has been problematic, the “bright defect” meaning that a light leaks from the peripheral part of liquid crystal display device so as to whiten the part.
In order to solve the above-mentioned problem, a pressure-sensitive adhesive has been proposed, that is configured to have a stress relaxing property by adding a low molecular weight polymer to a pressure-sensitive adhesive composition. For example, a pressure-sensitive adhesive composition for polarization plate has been disclosed, that comprises a high molecular weight acrylic copolymer, a low molecular weight acrylic copolymer having weight-average molecular weight of not more than 30,000 and a polyfunctional compound (for example, refer to Patent Literature 1). The pressure-sensitive adhesive composition described in Patent Literature 1 follows the dimensional change of polarization plate, so that occurrence of bright defect can be prevented. However, in the pressure-sensitive adhesive composition described in Patent Literature 1, an additive amount of the low molecular weight acrylic copolymer having weight-average molecular weight of not more than 30,000 is large, thus it is difficult to prevent occurrence of bubbles and separation under high temperature and humidity conditions.
In addition, a pressure-sensitive adhesive composition has been disclosed, that comprises a mixture of a low glass-transition temperature (Tg) acrylic copolymer having a content of a functional group containing monomer of not more than 0.5% by weight and a high Tg acrylic copolymer having a content of a functional group containing monomer of not less than 6% by weight, a crosslinking agent reactive with the functional group and an isocyanate compound, and that has a gel fraction of less than 30% by weight (for example, refer to Patent Literature 2). The pressure-sensitive adhesive composition described in Patent Literature 2 is capable of causing a cohesion force by forming a crosslinking structure in molecules of the high Tg acrylic copolymer, and restraining relation between molecules of the low Tg acrylic copolymer by a multimeric complex of the isocyanate compound. However, although the pressure-sensitive adhesive composition has molecules in which the crosslinking structure is hardly formed so that bright defect is scarcely caused, it has a low cohesion force under high temperature condition, thus it is difficult to prevent separation or occurrence of air bubbles in an evaluation of durability.
In addition, a pressure-sensitive adhesive composition has been disclosed, that comprises an acrylic copolymer having carboxyl group and hydroxyl group and an isocyanate compound added to the acrylic copolymer in an amount of 0.3 to 3 parts by weight (for example, refer to Patent Literature 3). The pressure-sensitive adhesive composition described in Patent Literature 3 has the above-mentioned constitution so that it is excellent in durability and inhibition of bright defect, if the pressure-sensitive adhesive composition is used for a liquid crystal display device having a small size as described in Patent Literature 3. However, if used for a liquid crystal display device having a large size, it is inferior in durability and inhibition of bright defect, thus a pressure-sensitive adhesive composition is needed, that has further excellent durability and is capable of inhibiting bright defect at a higher level.
Furthermore, recently, the optical film becomes also larger in size in accordance with the size increase of liquid crystal display device. Dimensional change such as shrinkage or expansion of the optical film becomes larger in accordance with the size increase, thus higher durability and inhibition of bright defect are needed. A pressure-sensitive adhesive composition having a high cohesion force has been disclosed as a pressure-sensitive adhesive composition satisfying durability even if used for a liquid crystal display device having a large display. For example, a pressure-sensitive adhesive composition for polarization plate has been disclosed, that comprises a mixture of an acrylic copolymer (A) having carboxyl-group or amide group but not having hydroxyl group and an acrylic copolymer (B) having carboxyl-group or amide group and having hydroxyl group, the copolymer (A) and copolymer (B) being mixed in a weight ratio ((A)/(B)) of 20/80 to 50/50 and an isocyanate compound blended to the mixture (for example, refer to Patent Literature 4). However, although the pressure-sensitive adhesive composition described in Patent Literature 4 is capable of preventing occurrence of bubbles and separation under high temperature and humidity conditions, it has insufficient stress relaxation properties, thereby bright defect easily occurs.